Ion cyclotron resonance (ICR) spectroscopy is well known and has been employed in numerous spectroscopy devices and studies. Ion cyclotron resonance techniques and devices provide sensitive and versatile means for analyzing ions.
ICR spectroscopy is based on the well known phenomenon that a charged particle having a velocity v moving through a uniform magnetic field describes a circular trajectory. Thus, the moving charged particle is constrained to move in circular orbits which lie in a plane which is perpendicular to the magnetic field. The motion of the charged particle in a direction of motion which is parallel to the direction of the magnetic field is unrestrained. The frequency of the charged particle's circular motion, known as the cyclotron frequency, is directly dependent upon the ratio of the particle's charge to its mass (charge-to-mass ratio) and the strength of the magnetic field. When the orbiting charged particles or ions are subjected to an oscillating electric field disposed at right angles to the magnetic field, those ions having a cyclotron frequency approximately equal to the frequency of the oscillating electric field are accelerated to increasingly larger orbital radii and increasingly higher kinetic energies. Ions having a cyclotron frequency substantially equal to the frequency of the oscillating electric field are said to be resonant with the electric field. Since only the resonant ions absorb energy from the oscillating electric field, they are distinguishable from non-resonant ions upon which the oscillating electric field has substantially no effect.
Various methods of and apparatus for taking advantage of the foregoing phenomena and utilizing it to measure the number of ions having a particular cyclotron frequency have been proposed and are in use. These devices are generally referred to as ion cyclotron resonance mass spectrometers.
In the omegatron type of ion cyclotron resonance mass spectrometer, gaseous ions are generated inside the device by bombardment of a gaseous sample with moving electrons. These ions are then subjected simultaneously to a magnetic field and an oscillating electric field which are mutually perpendicular. As described above, those ions having a cyclotron frequency which closely matches the frequency of the oscillating electric field, i.e., are in resonance with the frequency of the oscillating electric field, are accelerated to higher velocities and hence follow trajectories having increasingly larger orbital radii. The orbital radii of such resonant ions ultimately increase to a dimension at which the ions impinge upon a collector plate, and the resulting ion current is detected, measured and recorded. The mass spectrum of a sample to be analyzed may be scanned by varying either the frequency of the oscillating electric field or the strength of the magnetic field, or both, so as to bring ions of differing mass-to-charge ratios into resonance with the oscillating electric field.
In another type of ion cyclotron resonance mass spectrometer, ions having a cyclotron frequency equal to the frequency of the oscillating electric field are accelerated, and the resultant power absorbed from the electric field is measured. The measured absorbed power is related only to the resonant ions, and not to ions having other non-resonant cyclotron frequencies. Thus, detection of the absorbed power results in a measurement of the number of ions present in the sample which have the particular mass-to-charge ratio corresponding to the resonant frequency.
Obviously, a spectrum of ion mass-to-charge ratios for a particular ionized gas sample can be obtained by scanning a range of resonant frequencies and detecting the absorbed electric field power as a function of the resonant frequencies. An example of an ion cyclotron resonance mass spectrometer utilizing such a resonance absorption detecting technique is disclosed in U.S. Pat. No. 3,390,265 entitled "ION CYCLOTRON RESONANCE MASS SPECTROMETER HAVING MEANS FOR DETECTING THE ENERGY ABSORBED BY RESONANT IONS," issued to Peter M. Llewellyn on June 25, 1968.
Other U.S. patents disclosing various related ion cyclotron resonance mass spectrometer methods and apparatus, and improvements thereto, are: U.S. Pat. No. 3,446,957 entitled "ION CYCLOTRON RESONANCE SPECTROMETER EMPLOYING MEANS FOR RECORDING IONIZATION POTENTIALS", issued to David E. Gielow et al on May 27, 1969; U.S. Pat. No. 3,475,605 entitled "ION CYCLOTRON DOUBLE RESONANCE SPECTROMETER EMPLOYING A SERIES CONNECTION OF THE IRRADIATING AND OBSERVING RF SOURCES TO THE CELL" issued to Peter M. Llewellyn on Oct. 28, 1969; U.S. Pat. No. 3,502,867 entitled "METHOD AND APPARATUS FOR MEASURING ION INTERRELATIONSHIPS BY DOUBLE RESONANCE MASS SPECTROSCOPY", issued to Jesse L. Beauchamp on Mar. 24, 1970; U.S. Pat. No. 3,505,516 "ION CYCLOTRON RESONANCE SPECTROMETER EMPLOYING AN OPTICALLY TRANSPARENT ION COLLECTING ELECTRODE", issued to David E. Gielow et al on Apr. 7, 1970; U.S. Pat. No. 3,505,517 entitled "ION CYCLOTRON RESONANCE MASS SPECTROMETER WITH MEANS FOR IRRADIATING THE SAMPLE WITH OPTICAL RADIATION", issued to Peter M. Llewellyn on Apr. 7, 1970; U.S. Pat. No. 3,511,986 entitled "ION CYCLOTRON DOUBLE RESONANCE SPECTROMETER EMPLOYING RESONANCE IN THE ION SOURCE AND ANALYZER", issued to Peter M. Llewellyn on May 12, 1970; U.S. Pat. No. 3,535,512 entitled "DOUBLE RESONANCE ION CYCLOTRON MASS SPECTROMETER FOR STUDYING ION-MOLECULE REACTIONS", issued to John D. Baldeschwieler on Oct. 20, 1970; and U.S. Pat. No. 3,677,642 entitled "ION CYCLOTRON RESONANCE STIMULATED LOW-DISCHARGE METHOD AND APPARATUS FOR SPECTRAL ANALYSIS", issued to J. D. Baldeschwieler on July 18, 1972. In general, all of the foregoing patents disclose ion cyclotron resonance mass spectrometers which utilize multiple region analyzer cells and a resonance power absorption detection system which exposes the ions to an oscillating electric field.
A different type of ion cyclotron resonance mass spectrometer is disclosed in U.S. Pat. No. 3,742,212 entitled "METHOD AND APPARATUS FOR PULSED ION CYCLOTRON RESONANCE SPECTROSCOPY", issued to Robert T. McIver, Jr. on June 26, 1973. The spectrometer disclosed in this patent includes a single section trapped ion analyzer cell and a pulsed mode of operation. In this system, a gas sample is ionized within the cell by means such as a pulse of an electron beam. The ions are subjected to the combined action of a plurality of static electric fields and a magnetic field, thereby trapping the ions and causing them to move orbitally within the cell. After a known delay period, ions are detected by measuring the power they absorb from an oscillating electric field oriented perpendicular to the direction of the magnetic field. The ions are then removed from the cell by altering the voltages applied to the plates of the cell. The total operation sequence (ion formation, delay period, ion cyclotron resonance detection and ion removal) is then repeated. This apparatus provides much higher mass resolution than the omegatron and much longer ion trapping times than the multiple region cells used previously. A related apparatus which is capable of storing ions for several seconds is disclosed in U.S. Pat. No. 4,105,917 entitled "METHOD AND APPARATUS FOR MASS SPECTROMETRIC ANALYSIS AT ULTRA-LOW PRESSURES", issued to Robert T. McIver, Jr. and Edward B. Ledford, Jr. on Aug. 8, 1978.
One limitation of all the above-noted ion cyclotron resonance methods and apparatus is that ion cyclotron resonance detection is limited to a single frequency (and therefore a single mass-to-charge ratio) at any instant in time. In order to obtain a complete mass spectrum, it is necessary to scan either the magnetic field strength or the frequency of the oscillating electric field over a range of values so as to achieve resonance with the ions of various mass-to-charge ratios of interest. Typically, several minutes are required to completely scan the mass range of interest, and this severely limits the detection sensitivity of the spectrometer.
Conceptually similar problems are encountered in other forms of resonance spectroscopy, and Fourier transform techniques have been widely used to decrease the time needed to acquire data covering a broad spectrum and to enhance sensitivity. In general, Fourier transform techniques provide for the detection of a complete spectrum of information in the time normally needed to scan through one resonance element using conventional scanning techniques. In this regard, U.S. Pat. No. 3,475,680 entitled "IMPULSE RESONANCE SPECTROMETER INCLUDING A TIME AVERAGING COMPUTER AND FOURIER ANALYZER", issued to Weston A. Anderson and Richard Ernst on Oct. 28, 1969, discloses a nuclear magnetic resonance (NMR) spectrometer which includes a probe for containing a sample of matter to be analyzed, the sample being capable of having a plurality of different resonant groups. A radio frequency transmitter applies coherent oscillations to the sample. The coherent oscillations are modulated so that different resonance groups, at different resonant frequencies, are simultaneously excited thus producing a composite resonance signal which has a transient character. The composite transient resonance signal is detected in a receiver and fed to a time averaging computer and stored in a memory of the computer. The stored data is subsequently read out and Fourier analyzed to separate the different resonant components at the different resonant frequencies of the sample.
Specifically, the disclosed technique in the Anderson patent for simultaneously exciting a plurality of resonant frequencies comprises pulse modulating a 60 megacycle (MHz) sine wave excitation signal. The modulating pulse may be 100 microseconds in length and have a repetition rate of one cycle per second. While this technique is adequate for simultaneous excitation of multiple resonances in some types of resonance spectroscopies, including NMR, it has been found to be useful in ICR mass spectroscopy only for relatively narrow mass ranges.
Another method and apparatus for excitation of multiple resonances nearly simultaneously in magnetic resonance spectroscopy is described in U.S. Pat. No. 3,725,773 entitled "RF SPECTROMETER HAVING MEANS FOR EXCITING RF RESONANCE OF A PLURALITY OF RESONANCES LINES SIMULTANEOUSLY USING A HIGH SPEED SCANNING MEANS", issued to Forrest A. Nelson on Apr. 3, 1973. This patent discloses a Fourier transform spectrometer having a sample immersed in a polarizing magnetic field and irradiated with radio frequency energy. The frequency of the oscillator is rapidly and repetitively scanned over a range of values to repetitively excite resonance of a plurality of resonance lines within the sample. The scan repetition rate is sufficiently high to sustain simultaneous resonance of the plurality of excited resonance lines. A transient signal representative of the composite resonance signal emanating from the sample is complex multiplied by a signal representative of the scan frequency and then subjected to Fourier transform analysis to separate the individual resonances.
Other U.S. patents disclosing various resonance spectrometers and apparatus, and improvements thereto, are: U.S. Pat. No. 3,461,381 entitled "PHASE SENSITIVE ANALOG FOURIER ANALYZER READOUT FOR STORED IMPULSE RESONANCE SPECTRAL DATA" issued to Forrest A. Nelson et al on Aug. 12, 1969; U.S. Pat. No. 3,530,371 entitled "IMPULSE FIELD-FREQUENCY CONTROL FOR IMPULSE GYROMAGNETIC RESONANCE SPECTROMETERS" issued to Forrest A. Nelson et al on Sept. 22, 1970; U.S. Pat. No. 3,651,396 entitled "FOURIER TRANSFORM NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY" issued to Richard C. Hewitt, et al on Mar. 21, 1972; and U.S. Pat. No. 3,810,001 entitled "NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY EMPLOYING DIFFERENCE FREQUENCY MEASUREMENTS" issued to Richard R. Ernst on May 7, 1974. In general, all of these references disclose resonance spectrometers which use a radio frequency transmitter or oscillator to generate an alternating electromagnetic field which excites a plurality of resonance lines, detection of a composite resonance signal comprising signals representative of the plurality of resonances and Fourier transform means for separating the individual resonance lines.
Fourier transform techniques, similar to those previously described for resonant spectroscopies in general and in particular to NMR, may also be applied to mass spectroscopy. One such application, the Fourier transform ion cyclotron resonance method, has many important advantages over prior art ion cyclotron resonance mass spectrometers, including very high mass resolution, high mass measurement accuracy and rapid data acquisition.
One application of Fourier transform techniques to ion cyclotron resonance mass spectroscopy is disclosed in U.S. Pat. No. 3,937,955 entitled "FOURIER TRANSFORM ION CYCLOTRON RESONANCE SPECTROSCOPY AND METHOD", issued to Melvin B. Comisarow and Alan G. Marshall on Feb. 10, 1976. This patent discloses a Fourier transform ion cyclotron resonance method wherein gaseous ions in a single section ion cyclotron resonance cell are subjected to a pulsed broadband oscillating electric field disposed at right angles to a magnetic field. As the frequency of the applied electric field reaches the cyclotron frequency of various ions, those ions absorb energy from the field and accelerate on spiral paths to larger radius orbits. A broad range of masses may be excited nearly simultaneously by applying a scanned frequency electric field to the ions over a short period of time. Typically, the frequency of the applied electric field is scanned very rapidly using a computer-controlled frequency synthesizer to generate a "chirp" excitation signal. The chirp excitation used by Comisarow comprises a fast (ca. 1 ms) frequency sweep which varies linearly from a low frequency value to a high frequency value and has an amplitude of a few tens of volts. The chirp signal thus excites the entire predetermined bandwidth of cyclotron frequencies of ions in a few milliseconds. The excited cyclotron motion of the ions is then sensed and digitized in the time domain, and the resulting signal is Fourier transformed into the frequency domain to reveal the mass spectrum of ions in the cell.
Inherent in all swept frequency excitation techniques is the fact that the excitation of different resonances does not occur simultaneously, but only at the time the resonant frequency is present in the excitation signal. Additionally, the instrumentation required to produce chirp excitation for ICR mass spectroscopy is very sophisticated and expensive.
Another method of simultaneously exciting multiple resonances is the rf burst excitation technique. This technique is commonly used in NMR. However, rf burst excitation has been found to be inadequate for broad range mass spectroscopy. It was theorized in an article entitled "Theory of Fourier Transform Ion Cyclotron Resonance Mass Spectroscopy: Response to Frequency-sweep Excitation" by Alan G. Marshall and D. Christopher Roe, published in J. Chem. Phys. Vol. 73, No. 4, 1980, pp. 1581-1590, that simultaneous excitation of a broad mass range (from 15 to 500, corresponding to cyclotron frequencies from 50 kHz to 2 MHz at 2 Tesla) with the rf burst method would require an rf burst excitation signal having a duration of about 30 nanoseconds and an amplitude of 13,200 volts. Since it was and still is extremely impractical to create such a signal, this approach was abandoned in favor of the above described frequency sweep chirp excitation.
A further advancement in ion cyclotron resonance mass spectroscopy is disclosed in U.S. Pat. No. 4,535,235 entitled "APPARATUS AND METHOD FOR INJECTION OF IONS INTO AN ION CYCLOTRON RESONANCE CELL," issued to Robert T. McIver, Jr. on Aug. 13, 1985. The spectrometer disclosed in this patent is more versatile than those previously developed because the ionizer for forming ions is outside the magnetic field and separate from the ion cyclotron resonance cell. Placing the ionizer outside of the magnetic field permits a wide variety of methods to be used to form ions from a sample. The ions are transported by a quadrupole mass filter through the fringing fields of the magnet and are injected into an ion cyclotron resonance cell that is disposed in the homogeneous region of the field. Once the ions are in the cell, they are accelerated and mass analyzed using either the methods of Fourier transform ion cyclotron resonance or ion cyclotron resonance power absorption.
A recent development in Fourier transform mass spectroscopy is described in an article entitled "Parametric Mode Operation of a Hyperbolic Penning Trap for Fourier Transform Mass Spectrometry" by D. L. Rempel, E. B. Ledford, Jr., S. K. Huang and M. L. Gross, published in Analytical Chemistry, Vol. 59, No. 20, pp. 2527-2532 (1987). Described in this article is a system wherein the static electric and magnetic fields of a hyperbolic Penning trap form a cell having fields which are similar to those in a single region ion cyclotron resonance cell. However, instead of six flat electrodes, as disclosed in previously discussed U.S. Pat. No. 3,742,212 issued to Robert T. McIver, Jr., the hyperbolic Penning trap comprises three electrodes, two "end caps" and one "ring" electrode, which are hyperbolas of revolution. Usable cyclotron resonance signals were obtained with this device by applying a near critically damped sinusoidal signal between the end caps and the ring electrode. The signal used for ion excitation has a peak of approximately +80 volts and a positive voltage duration of approximately 1.55 microseconds followed by a negative voltage portion having a peak of approximately -6.4 volts. However, the authors report that the tuning behavior of the Penning trap is unexpectedly sensitive to the trap voltage and the amplitude of the excitation signal. Furthermore, they suggest that this method can excite the z-axial mode sufficiently to cause ions to be ejected from the cell.
Although there are many advantageous features of the Fourier transform ion cyclotron resonance method, a number of problems and limitations remain. One disadvantage is that the computer-controlled frequency synthesizer, which is used to generate the pulsed broadband oscillating electric field, i.e. frequency chirp, is complex and expensive. Typically, it must be capable of scanning a frequency range of several megahertz in a time period of just a few milliseconds. In addition, the synthesizer must be highly stable and reproducible from scan to scan so that repetitive scans can be summed together coherently to improve the signal-to-noise ratio of the measurement.
Another disadvantage of the above described Fourier transform ion cyclotron resonance spectroscopy techniques is that ions of different mass are accelerated at different times as the frequency of the oscillating electric field is scanned. This complicates the Fourier transform analysis because ions of different mass have different initial phase angles for their cyclotron motion. Correcting the phase angle problem is further complicated by phase shifts in the signal amplifiers. The problem is so complex that most Fourier transform ion cyclotron resonance spectrometers present only a magnitude mode spectrum, which is a composite of the real and imaginary components which result from the Fourier transform analysis. This procedure produces a significantly broader line shape and degrades the mass resolution of the spectrometer by about a factor of 2.
Many of the deficiencies found in presently used resonance spectrometer systems could be overcome with a system which simultaneously excites all of the resonant components. At the same time, the system should be approximately equally sensitive to all of the resonant components. Such a system should not be overly sensitive to other system parameters. It is also desireable that the system be of simple construction, adaptable to a variety of resonance spectrometer configurations and cost effective. A need thus exists for a system which excites all ions in a short time interval, less than a microsecond so as to more closely approximate the ideal situation of a delta function acceleration of the ions. Additionally, the system should provide more stable peak heights and better isotope ratios when used in Fourier transform mass spectroscopy. The present invention overcomes these and other short comings of the prior art by providing a new and improved method and apparatus for impulse acceleration of ions which is more sensitive, provides better resolution, is less complex and less expensive than other broadband excitation methods disclosed in the prior art.